1. Field of the Invention
The invention relates to a high-crystallinity garnet-type solid electrolyte, a secondary battery that contains the garnet-type solid electrolyte, and a method of producing the garnet-type solid electrolyte.
2. Description of Related Art
A secondary battery is a battery that can undergo discharge by converting a decrement in chemical energy accompanying a chemical reaction into electrical energy and that can engage in storage (charging) by converting electrical energy into chemical energy when current flow occurs in the reverse direction from that during discharge. Among secondary batteries, the secondary batteries typified by lithium secondary batteries are, due to their high energy densities, widely used as power sources for notebook and laptop computers, portable phones, and so forth.
The reaction given by equation (I) below occurs at the negative electrode during the discharge of a lithium secondary battery when graphite (indicated below by C) is used as the negative electrode active material.LixC→C+xLi++xe−  (I)(0<x<1 in equation (I))The electrons produced by the reaction according to equation (I) travel through an external circuit and do work at an external load and thereafter reach the positive electrode. The lithium ion (Li+) produced by the reaction according to equation (I) migrates by electroosmosis from the negative electrode side to the positive electrode side within the electrolyte interposed between the negative electrode and positive electrode.
The reaction given by equation (II) below occurs at the positive electrode during discharge when lithium cobaltate (Li1−xCoO2) is used as the positive electrode active material.Lii−xCoO2+xLi++xe−→LiCoO2   (II)(0<x<1 in equation (II))During charging, the reverse reactions to equations (I) and (II) occur at the negative electrode and positive electrode, respectively, reproducing the lithium-loaded graphite (LixC) at the negative electrode by graphite intercalation and reproducing the lithium cobaltate (Li1−x,CoO2) at the positive electrode and thus enabling discharge to occur again.
Among secondary batteries, all-solid-state secondary batteries, in which the electrodes and electrolyte are all solids, have been the subject of very active investigations in the last few years. And among all-solid-state secondary batteries, attention has been focused on the use of garnet-type ceramic materials as the solid electrolyte in all-solid-state lithium secondary batteries. Japanese Patent Application Publication No. 2010-045019 (JP-A-2010-045019) describes art related to an all-solid-state lithium secondary battery that includes a positive electrode, a negative electrode, and a solid electrolyte that contains a ceramic that has a garnet-type or garnet-like crystalline structure composed of Li, La, Zr, and O.